1. Field of the Invention
The present invention relates to surface-emitting-type semiconductor lasers that emit laser beams perpendicularly to semiconductive substrates.
2. Related Art
Semiconductor lasers are classified into edge-emitting-type semiconductor lasers that emit laser beams from cleavage planes and surface-emitting-type semiconductor lasers that emit laser beams perpendicularly to semiconductive substrates. In semiconductor lasers, an active layer having a gain with respect to light is disposed in a resonator provided between mirrors so that light moving back and forth in the resonator is amplified until oscillation occurs. Edge-emitting-type semiconductor lasers have been widely used since they have simplified configurations having mirrors as cleavage planes and can produce high-output laser beams. On the other hand, surface-emitting-type semiconductor lasers have semiconductive multilayered mirrors or dielectric mirrors. Thus, they have complicated configurations; however, they have the following advantages: (1) low threshold currents, (2) ability to be arrayed on semiconductive substrates, (3) a single mode of longitudinal oscillation, (4) stable oscillation wavelength, and (5) circular (conical) beams being obtainable.
Surface-emitting-type semiconductor lasers, however, have a problem of difficult control of polarization planes. Since an edge-emitting-type semiconductor laser has a resonator consisting of a waveguide, TE waves have larger reflectance compared with TM waves on the end faces of the waveguide, and thus the TE waves having an electric field vector parallel to a semiconductive substrate will oscillate. Light emerging from the edge-emitting-type semiconductor laser has a stable polarization plane without fluctuation. In contrast, in a surface-emitting-type semiconductor laser, it is difficult to enhance the reflectance of mirrors with respect to light polarized in a specific direction and to raise the gain of the activation layer. Since the surface-emitting-type semiconductor laser has an isotropic configuration with respect to polarized light, it has problems such as fluctuation and instability of the polarization direction. Reflectance of most beam splitters and diffraction gratings depends on the polarization direction, hence fluctuation of the polarization direction hinders use of the semiconductor laser when it is mounted in an optical apparatus. Furthermore, an unstable polarization plane causes irregular movement of the orthogonal polarization directions on the polarization plane, which will generate noise.
The following are countermeasures for solving this problem. In a first method, thin metallic lines are arranged in one direction on a semiconductive multilayered mirror to increase the reflectance of the mirror with respect to the polarized light in a specific direction. Since the reflectance increases with respect to light having a polarization direction parallel to the thin metallic lines, this method is effective to some extent for stabilization of the polarization plane; however, the thin metallic lines must have a size which is less than the wavelength of light; hence it is difficult to form them. An alternative method uses dependence of the gain on crystal orientation, in which an active layer is formed on a high-index oriented crystal plane, such as a (311)A plane or a (311)B plane, to raise the gain of the active layer with respect to polarized light in a predetermined direction. This method, however, has disadvantages, such as difficulty in growing crystals and in obtaining high output.
In addition, an attempted method is control of polarized light by a resonator having a specified shape. It has been found that a rectangular resonator facilitates orientation of polarized light in the short or long side direction, and this fact suggests possibility of ready control of the polarized light. Its cause, however, has not been clarified, and the method has less reproducibility.
Accordingly, a surface-emitting-type semiconductor laser in accordance with the present invention includes at least one strain generating section adjacent to a resonator. This configuration allows control of orientation in a specific direction of the polarization plane of light emerging from the opening of the resonator; hence the orientation of the polarized light is stabilized regardless of changes in the operating environment. Furthermore, the orientation of the polarized light is not irregularly switched, and thus noise is not generated. An effective configuration includes two strain generating sections disposed on a straight line which passes through the center of the resonator, so as to sandwich the resonator. The polarization plane is thereby stabilized in a direction parallel to or perpendicular to the straight line passing through the strain generating sections.
An effective distance between the boundary of the resonator and the boundary of each strain generating section for controlling the polarization plane is in a range of 0 xcexcm to 100 xcexcm. Herein, xe2x80x9c0 xcexcmxe2x80x9d means that the resonator can come into contact with the strain generating section.
The strain generating section is composed of a metal, a dielectric material, or a semiconductor formed so as to be bonded to a semiconductor continuously extending from the resonator. This enables control of the polarization plane without complication of the production process. Alternatively, it may be composed of a dielectric material or a semiconductor formed in a semiconductor continuously extending from the resonator. In this case, the strain generating section can also be formed during the production process of the resonator; hence effective control of the polarization plane can be performed without increased production cost.
It is preferable that the main component of the dielectric material formed in the semiconductor be aluminum oxide, because it can be readily formed by selective oxidation of the AlAs layer in the production process. Since it can prevent an excess current in the strain generating section, heating of the device is reduced and electrical power consumed is reduced as much as possible. Complete oxidation of the AlAs layer is not necessary as long as the area of the residual AlAs layer is 9 xcexcm2 or less. In an AlAs layer having such an area, the effects of the excess current are not significant. When the AlAs layer remains in the strain generating section, more satisfactorily results will be obtained in view of reliability of the device.
The strain generating section is preferably an indented section which is provided on a semiconductor continuously extending from the resonator. Such a configuration can effectively control polarization when the distance between the surface of the semiconductive layer and the activation layer is relatively large, as in a resonator formed by proton implantation.
Preferably, a plurality of resonators having strain generating sections are provided on a single semiconductive piece, and directions of arrangement of the resonators and the strain generating sections are different from each other. Laser beams having different polarization directions can thereby emerge from one semiconductive piece.